Chiral Ru-based complexes for asymmetric olefin metathesis: enhancement of catalyst activity through steric and electronic modifications

Design, synthesis, characterization, and catalytic activity of six enantiomerically pure Ru-based metathesis catalysts are disclosed (3a-3f). The new chiral catalysts were prepared through steric and electronic alterations of the parent catalyst system (3). The present studies indicate that the effect of structural modifications of chiral complex 3 does not always correspond to those of the related achiral complexes. The present findings illustrate that modified Ru complexes (3e and 3f) deliver reactivity levels that are more than 2 orders of magnitude higher than 3. Reactivity and physical data are provided that shed light on the origin of activity differences. Some members of the new generation of chiral Ru catalysts promote asymmetric ring-opening (AROM) and ring-closing (ARCM) metatheses that cannot be effected by the first generation chiral catalyst (3).     

Measurements of electron-transfer rates of charge-storage molecular monolayers on Si(100). Toward hybrid molecular/semiconductor information storage devices

Redox kinetics were measured for two electroactive molecules attached to Si(100) surfaces, a ferrocene (Fc-BzOH) and a Zn(II) trimesitylporphyrin (Por-BzOH). Each molecule was derivatized with a benzyl alcohol linker for attachment to the Si surface via the formation of a Si-O bond. A complete protocol was developed for the preparation of stable Si(100) surfaces derivatized with the electroactive molecules. The redox-kinetic measurements were performed on the resulting Fc-BzOH and Por-BzOH monolayers to probe (1) the rate of electron transfer (k0) for oxidation in the presence of applied potentials and (2) the rate of charge dissipation after the applied potential is disconnected (in the form of a charge-retention half-life t1/2). The k0 values for the two types of monolayers were found to be similar to one another as were the t1/2 values. Perhaps more importantly, the electron-transfer rates for both the Fc-BzOH and the Por-BzOH monolayers differ from the charge-dissipation rates by approximately 6 orders of magnitude and are strongly dependent on the surface concentration of the electroactive species. For the Por-BzOH monolayers on Si(100), the k0 and t1/2 values and their trends as a function of surface coverage were determined to be similar to those previously measured for the analogous thiol-derivatized molecule assembled on Au(111). In contrast, the Fc-BzOH monolayers on Si(100) were found to exhibit much slower electron-transfer and charge-dissipation rates than those in the corresponding thiol-Au(111) case. Two alternative hypotheses are advanced to explain both the diminution in rates with increased surface coverage and the contrasting behavior with the analogous thiols on Au, one based on space-charge effects at the monolayer-solution interface, and a second relying on changes in distance of the redox centers from the surface as modulated by the orientation of the linking chains. Collectively, the ability to prepare and study stable, electroactive molecular media on Si(100) is likely to be key in the development of hybrid molecular/semiconductor devices.     

An enantiomerically pure adamantylimido molybdenum alkylidene complex. An effective new catalyst for enantioselective olefin metathesis

An enantiomerically pure Mo-based complex that bears an alkylimido ligand is prepared and characterized through NMR spectroscopy and X-ray analysis. Mo complex 4 is the only reported chiral alkylimido catalyst; all previous chiral complexes are arylimido systems. These studies show that the chiral Mo catalyst exists exclusively as the syn isomer and that it offers unique reactivity and selectivity profiles in asymmetric olefin metathesis.     

Enantioselective synthesis of propargylamines through Zr-catalyzed addition of mixed alkynylzinc reagents to arylimines

[reaction: see text] Addition of mixed alkynylzinc reagents to various arylimines is catalyzed by chiral amino acid-based ligand 1 and Zr(Oi-Pr)(4).HOi-Pr to afford chiral propargylamines in up to 90% ee. Oxidative removal of the o-anisidyl group affords the free amine, which can then be acylated.     

Thin points for Brownian motion

Let Θ(x,r) denote the occupation measure of the ball of radius r centered at x for Brownian motion {W_t}_0≤t≤1 in . We prove that for any analytic set E in [0,1], we have

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, where dim_P(E) is the packing dimension of E. We deduce that for any a≥1, the Hausdorff dimension of the set of “thin points” x for which

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, is almost surely 2−2/a; this is the correct scaling to obtain a nondegenerate “multifractal spectrum” for the “thin” part of Brownian occupation measure. The methods of this paper differ considerably from those of our work on Brownian thick points, due to the high degree of correlation in the present case. To prove our results, we establish general criteria for determining which deterministic sets are hit by random fractals of ‘limsup type' in the presence of long-range correlations. The hitting criteria then yield lower bounds on Hausdorff dimension. This refines previous work of Khoshnevisan, Xiao and the second author, that required decay of correlations.     

Simple Analytical Model of the Anodic Streamer

In this work, an analytical model is used to study the formation of the anodic streamer in high pressure electrical discharge. This model enabled us to see the space variations of the characteristics of the streamer such as the electric field and the propagation velocity of streamer. The validity of the analytic approach is demonstrated by comparing the model results to the data from the literature. A qualitative concord was found. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Crystallization and stereocomplexation behavior of poly(D‐ and L‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) block copolymers

The thermal properties, crystallization, and morphology of amphiphilic poly(D ‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) (PDLA‐b‐PDMAEMA) and poly (L ‐lactide)‐b‐poly(N,N‐dimethylamino‐2‐ethyl methacrylate) (PLLA‐b‐PDMAEMA) copolymers were studied and compared to those of the corresponding poly(lactide) homopolymers. Additionally, stereocomplexation of these copolymers was studied. The crystallization kinetics of the PLA blocks was retarded by the presence of the PDMAEMA block. The studied copolymers were found to be miscible in the melt and the glassy state. The Avrami theory was able to predict the entire crystallization range of the PLA isothermal overall crystallization. The melting points of PLDA/PLLA and PLA/PLA‐b‐PDMAEMA stereocomplexes were higher than those formed by copolymer mixtures. This indicates that the PDMAEMA block is influencing the stability of the stereocomplex structures. For the low molecular weight samples, the stereocomplexes particles exhibited a conventional disk‐shape structure and, for high molecular weight samples, the particles displayed unusual star‐like shape morphology. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 1397–1409, 2011     

Curvilinear smoothed particle hydrodynamics

We suggest a new set of equations to employ smoothed particle hydrodynamics (SPH) in a curvilinear space, and we refer to it as curvSPH. In classical SPH, the horizontal and vertical resolution of discretization is supposed to be equal for fluid particles. However, curvSPH makes the horizontal and vertical resolutions independent from each other. This is performed by transformation of physical space into an appropriate computational space with a different scale in horizontal and vertical directions. Solving a problem using SPH in a curvilinear space also provides capability to model curved boundaries as straight lines. In classical SPH, special care is needed to reach a uniform mass distribution along curved boundaries; however, producing uniform mass distribution along a line using curvSPH is straight forward. Different simulations, including simulation of a flip bucket are performed to demonstrate the applicability of the proposed method. Good agreement of results with experimental data and classical SPH confirms the capabilities of curvSPH. Copyright © 2016 John Wiley & Sons, Ltd.     

Building an Iron Chromophore Incorporating Prussian Blue Analogue for Photoelectrochemical Water Oxidation

The replacement of traditional ruthenium-based photosensitizers with low-cost and abundant iron analogs is a key step for the advancement of scalable and sustainable dye-sensitized water splitting cells. In this proof-of-concept study, a pyridinium ligand coordinated pentacyanoferrate(II) chromophore is used to construct a cyanide-based CoFe extended bulk framework, in which the iron photosensitizer units are connected to cobalt water oxidation catalytic sites through cyanide linkers. The iron-sensitized photoanode exhibits exceptional stability for at least 5 h at pH 7 and features its photosensitizing ability with an incident photon-to-current conversion capacity up to 500 nm with nanosecond scale excited state lifetime. Ultrafast transient absorption and computational studies reveal that iron and cobalt sites mutually support each other for charge separation via short bridging cyanide groups and for injection to the semiconductor in our proof-of-concept photoelectrochemical device. The reorganization of the excited states due to the mixing of electronic states of metal-based orbitals subsequently tailor the electron transfer cascade during the photoelectrochemical process. This breakthrough in chromophore-catalyst assemblies will spark interest in dye-sensitization with robust bulk systems for photoconversion applications.     

An effective ant colony optimization algorithm (ACO) for multi-objective resource allocation problem (MORAP)

The multi-objective resource allocation problem (MORAP) addresses the important issue which seeks to find the expected objectives by allocating the limited amount of resource to various activates. Resources may be manpower, assets, raw material or anything else in limited supply which can be used to accomplish the goals. The goals may be objectives (i.e., minimizing costs, or maximizing efficiency) usually driven by specific future needs. In this paper, in order to obtain a set of Pareto solution efficiently, we proposed a modified version of ant colony optimization (ACO), in this algorithm we try to increase the efficiency of algorithm by increasing the learning of ants. Effectiveness and efficiency of proposed algorithm was validated by comparing the result of ACO with hybrid genetic algorithm (hGA) which was applied to MORAP later.